Nowadays, on the Internet of Thing (IoT) era, the demand for electricity in daily life is increasing at an unprecedented level with the popularization of portable electric and electronic devices. Normally, the source of this electricity usually comes from conventional fossil fuels, and the biggest concern is it limited resources and non-renewable as well as environmental issues that arise from the carbon footprints of fossil fuel usage. Therefore, enormous efforts have been conducted to replace conventional fossil fuel with renewable energy sources such as solar energy, wind energy, hydroelectric energy and so on. Amongst various energy harvesters, the mechanical energy harvester shows tremendous promise as a new generation energy harvester that is capable of harvesting energy around the clock.
After the first introduction of triboelectric generator (TENGs) in 2012 by the Wang [1] group that successfully reported the piezoelectric nanogenerators, it has attracted growing interest as an alternative method for scavenging the ambient mechanical energy in the environment to electricity. A TENGs comprise rubbing together two different triboelectric materials (either organic or inorganic materials) to initiate the triboelectrification process, followed by charge separation. TENGs are relatively cheaper as compared to piezoelectric nanogenerators (PENGs) and it also capable of producing higher output power and ease of fabrication [2]–[5]. TENGs are excellent candidates for the potential application in integrated energy harvesting devices and convert untapped mechanical energies directly to electrical signals from various sources such as wind flow, ocean waves, human motion and even blood movement inside human veins [6]–[10]. Since then, various triboelectric polymeric materials had been used in TENGs, such as polymers (polyamide, polytetrafluoroethylene [PTFE], polydimethylsiloxane [PDMS], polyvinylidene fluoride [PVDF]), [11]–[13]. For the realization of TENGs applications as energy harvesters, the demands of TENGs exhibit the following criteria is crucial such as high flexibility, ability to maximize electrical output and robustness in enduring high mechanical stress/strain. Even though typical TENGs consisting of the above tribo-materials is preferred due to the flexibility and good triboelectricity of the polymers, the polymer based TENGs still face critical issues like the inclination of the electron to recombined with positive charges induced on an electrode and low conductivity of the triboelectric polymers.
Poly(dimethylsiloxane) (PDMS) is one of the most negative triboelectric materials frequently used in TENGs because of its ability to gain electrons while maintaining high flexibility, high electronegativity, nontoxicity, and biocompatibility. Due to its simple preparation condition and physical tunability, PDMS has become a suitable choice for engineering properties in achieving the high-performance TENGs. In order to increase the electronic characteristics of PDMS-based TENG, incorporating high dielectric inorganic materials into a PDMS matrix could promote the relative permittivity and charge density of tribo-materials, which further boost the PDMS-based TENG electrical output[11]–[16]. Various type of high dielectric constant inorganics materials such as TiO2, SrTiO3, BaTiO3 (BTO), ZnSnO3 [17]–[19] were added into PDMS to increase the effective ε of the composite film. Amongst all of the dielectric materials, BTO is one of the most appealing due to its high dielectric constant and ferroelectric properties of the BTO making it an ideal candidate for nanogenerators [20].
Graphene has been widely utilized in diverse electrical devices and constructs. In a graphene monolayer, each carbon atom is covalently bonded with other nearby atoms to construct a honeycomb-like lattice. The essential characteristic of graphene is its excellent thermal and electrical conductivity. Graphene has been reported to exhibit an electrostatic behaviour and can store the electric charge when friction is applied which opens the doors for the potential application of graphene in TENGs [21]. Graphene is able to offer rich charge transfer pathways in triboelectric nanogenerators and lead to significant improvement in the output performance of the TENGs [22].
The reports of dispersed graphene quantum dots (GQD) as the filler to increase the dielectric constant of polymer materials are also being carried out as an alternative candidate besides the graphene and graphene oxides [23]. Its unique optical, electronic, spintronic, and photoelectric properties induced by the quantum confinement effect and edge effect as well as its fragments limited in size, or domains, of a single-layer two-dimensional layered structure with a large aspect ratio which allows GQD to form a large number of parallel micro-capacitors within the polymer matrix [24]–[26].
In this study, a high-performance composite-based triboelectric nanogenerator (CTENG) device, based on PDMS as polymeric matrix with BTO nanoparticles as dielectric fillers and graphene as conductive media was fabricated. The three-phase nanocomposite is observed to exhibit a percolation system, in which BTO were uniformly and randomly dispersed in the polymeric matrix and surrounded by graphene which formed discrete micro-capacitor structures. The highest voltage, current, and power density-producing PDMS/BTO/GQD nanocomposite will be found to have the best output performance. The stable and good electrical output power generated by the TENGs suggests that the device has the potential for energy harvesting in nano-energy applications.